Pressure Relief Valves - How They Work

Pressure relief valves are essential to keeping hydraulic and pneumatic systems below the set pressure. Depending on the installation, they can either:

• Reduce the downstream pressure to a constant level whenever it exceeds a threshold
• Maintain sustained pressures down- or upstream from the valve
• Reduce the peaks or pressure pulses to protect downstream equipment

This article is a comprehensive overview of pressure relief valves that explains their design, operation principle, applications, and how to install them in a system.

Table of contents

• Pressure relief valve design and operation
• Pressure relief valve applications
• Differences between pressure relief valves and safety valves
• Selection criteria
• Pressure relief valve installation
• FAQs

Pressure relief valve design and operation

Pressure relief valve design and operation varies among different types of relief valves. This section covers the design and operation of the following relief valves:

• Direct-acting pressure relief valve
• Balanced pilot-operated pressure relief valve
• Remote operated pressure relief valve
• Electric pressure relief valve

Direct-acting pressure relief valve

As seen in Figure 2 (left), a direct-acting pressure valve has an inlet, outlet, and a flow control mechanism (i.e., poppet, ball, or diaphragm) supported by an adjustable spring. Pressure relief valves have two designs for adjusting the spring: external and internal. An external design allows adjustment of the spring with a knob or handle that bolts onto the valve's outside. An internal design has a nut or screw within the valve's housing that turns in either direction to loosen or stiffen the spring. The internal design requires partial valve disassembly to adjust the spring's stiffness.

A direct acting pressure relief valve limits pressure in a system rising above a set limit. The spring (Figure 2 labeled A) in the valve, which is either adjustable or not, determines the limit. If the system's media pressure is not high enough, it will not open the valve’s opening mechanism (Figure 2 labeled B), which is typically a disc or poppet.

The opening mechanism begins to open when the media pressure reaches the valve’s cracking pressure. The relief valve discharges some media into the air, in the case of air compressors or a tank, in the case of hydraulic systems. This slows down how fast pressure builds in the system.

Finally, when the system's pressure reaches the relief valve’s set pressure limit, the valve is fully open, and all media discharges through the valve. This stops the operation of any downstream components until pressure reduces enough for the valve to begin closing. Known as the pressure override, the difference between the valve's cracking pressure and true relief valve setting is typically at least 13.8 bar (200 psi).

Balanced pilot-operated pressure relief valve

A balanced pilot-operated relief valve has an inlet, outlet, and two poppets: the main poppet and a bolted-on pilot poppet. Both poppets are spring-supported. A soft, non-adjustable soft spring supports the main poppet, and a much stiffer, adjustable spring supports the pilot poppet. A pilot hole lets media flow from the inlet to above the main poppet and to the pilot poppet. Also, media flows from the inlet to below the main poppet. When the main poppet rises, media from below flows towards the outlet.

Due to media being above and below the main poppet, and the poppet’s top and bottom having approximately the same surface area, the pressure on each side of the poppet is equal. This is how the soft spring can keep the poppet closed against high system pressures.

The pilot poppet is a direct-acting poppet that determines the relief valve’s pressure setting. When the system's pressure rises enough to crack the pilot poppet, media above the main poppet can flow past the pilot poppet to the outlet. This causes a pressure drop above the main poppet, allowing it to open so media below it can flow to the valve's outlet.

The benefit of a balanced pilot-operated pressure relief valve is the pressure override (the difference between the valve's cracking and set pressure limits) is much smaller than with a direct acting pressure relief valve. A smaller pressure override prevents unnecessary heat buildup and reduces energy consumption in a hydraulic system.

Remote operated pressure relief valve

A remote operated pressure relief valve is the same as a balance pilot-operated pressure relief valve except for one difference. This relief valve has a vent that connects to a direct-acting relief valve via a long, narrow hose. The direct-acting relief valve is typically a significant distance away, conveniently located on an operator’s console.

Media in a system will always travel the path of least resistance. So, when an operator adjusts the remote relief valve’s pressure setting to below the bolted pilot poppet’s pressure setting, the remote valve takes control of adjusting the system’s max pressure setting. Control returns to the pilot poppet when the operator adjusts the remote relief valve’s pressure setting to above the pilot poppet’s.

Electric pressure relief valve

An electric pressure relief valve system uses a remote operated relief valve that connects to multiple direct-acting relief valves. Each direct-acting relief valve connects to a solenoid valve that controls whether or not media can flow to the relief valve.

The direct-acting relief valves have pressure settings that are lower than the main pressure relief valve. For example, three direct-acting relief valves may have pressure settings of 1000, 2000, and 3000 psi (70, 138, and 207 bar) respectively. The main pressure relief valve may have a pressure setting of 4000 psi. When a solenoid turns on and opens, the media will follow the path of least resistance and flow towards the relief valve with the lowest setting. This setup allows a system to cycle through multiple pressure settings quickly–for example, a press machine in a manufacturing plant.

Pressure relief valve applications

Avoiding cavitation

Cavitation occurs when the pressure in a liquid drops rapidly below the vapor pressure. If a centrifugal pump is pumping against a closed system, overpressure needs to discharge within the pump housing. This causes areas of low pressure, potentially causing cavitation.

By opening proportionally with pressure increases, relief valves bypass the housing. This slowly discharges the excess pressure. And by avoiding cavitation, it increases the lifespan of the pump. Learn more by reading our guides on cavitation and flashing and cavitation in pumps, valves, and pipes.

Cooling/heating circuits

Flows may vary significantly with intermittent loads or during startup or shutdown. Reactions from connected boilers or reactors may cause pressure to rise or fall disproportionately to the input that user-controlled equipment (e.g., pumps and heat exchangers) generates. Pressure relief valves help avoid unexpected pressure changes in heating circuits.

Systems with sensitive equipment

Similarly, in pneumatic systems with multiple components, excessive pressure may damage equipment. A pressure relief valve can avoid premature equipment failure and can be part of a preventative maintenance plan.

Learn how a relief valve is suitable for swimming pool applications alongside a pool filter pressure gauge.

Differences between pressure relief valves and safety valves

Pressure relief valves have a continuous, proportional operation (Figure 3 labeled 1), whereas safety valves do not (Figure 3 labeled 2). Pressure relief valves are an operational part of fluid systems rather than only a safety feature. During normal operation, it's normal to have a continuously opened overflow/pressure relief valve to help the system rebalance to normal working conditions.

Safety valves, on the other hand, must remove all energy that may enter a system to prevent system failure. After the initial lifting of the disc, the area on which pressure acts enlarges, and the force on the spring increases significantly. The significant force causes the safety valve's popping characteristic.

Rather than discharging into a low-pressure zone of the circuit, safety valves discharge into the atmosphere and continue to do so even a short while after the system's pressure returns to threshold pressure. Given that safety valves protect pressure vessels, they do not stop discharging until reaching a safe situation.

The operation difference between a relief valve and a safety valve is observable in their respective pressure characteristics. These valves alternate between fully open (Figure 3 labeled A) and fully closed (Figure 3 labeled B).

• Blowdown: A relief valve’s blowdown (Figure 3 labeled F) is much narrower than a safety valve’s (Figure 3 labeled G). This means there is a much smaller difference between the set pressure and closing pressure, meaning it closes faster.
• Reseating pressure: The reseating pressure (Figure 3 labeled C), also known as the closing pressure, is the pressure at which the valve closes. Relief valves close proportionally, near the set pressure (Figure 3 labeled D), whereas safety valves close further from the set pressure.
• Maximum relieving pressure: Relief valves begin to close near the maximum relieving pressure (Figure 3 labeled E), whereas safety valves close much further from the maximum relieving pressure.
• Simmering: Safety valves have a wide simmering value (figure 3 labeled H), which is the buildup to the valve popping open.

Selection criteria

When selecting a pressure relief valve, consider the following criteria. Read our selecting relief valves and safety valves guide for more information.

• Minimum/maximum operating pressure: Ensure the relief valve is compatible with the system’s pressure limits.
• Body & seal materials: Ensure the relief valve components are compatible with the media.
• Discharge flow: Ensure the relief valve is the proper size to sufficiently discharge during an extreme scenario.
• Adjustable/non-adjustable: An adjustable relief valve is advantageous if the desired set pressure is unknown or possibly changing during the valve’s lifetime.

Pressure relief valve installation

Pressure relief valves work best downstream of high-pressure zones in a system. A common application is installing a relief valve near a pump’s discharge. Read our guide on installing relief valves and safety valves for more information.

FAQs

Does a pressure relief valve reduce flow?

Yes, but this only occurs when pressure exceeds the set pressure. During normal operation, pressures should not exceed the set level, and the relief valve remains closed.

What do you need to look for when selecting a pressure relief valve?

Maximum flow, maximum pressure, and the nature of the medium. Corrosive media might require different seals, diaphragm, or body. Ensure that the valve has the needed approvals for your application.

Can pressure relief valves prevent water hammer?

Yes - certain models can. To fully avoid water hammer, use a specific model that can accommodate both the high flow and pressure differentials common in a water hammer scenario.

Are there disadvantages to using pressure relief/overflow valves in a bypass?

The biggest disadvantage is the loss of energy. Everything that is being pumped through the valve is by definition not being used downstream. For short bursts, this is generally not an issue.